1. Field of the Invention
The present invention relates to a wide-band, high-yield surface acoustic wave (SAW) device including a SAW resonator connected in series to a SAW filter. The present invention also relates to a communication apparatus including such a SAW device.
2. Description of the Related Art
In communication systems for use in communication apparatuses, such as cellular telephones, the frequency band is increasing due to an increase in the number of subscribers and the diversity of services. Accordingly, there is a growing demand to increase the pass band of SAW devices provided with a SAW filter, which are widely used as RF bandpass filters in communication apparatuses, such as cellular telephones.
The number of systems having a transmission frequency band and a reception frequency band that are similar is increasing. In a transmission bandpass filter, the attenuation of the reception frequency band must be increased, and in a reception bandpass filter, the attenuation of the transmission frequency band must be increased. Accordingly, there is also a demand to increase the attenuation in a frequency band very close to the pass band.
A known configuration and a method for increasing the attenuation at the high-frequency side very close to the pass band is disclosed in Japanese Unexamined Patent Application Publication No. 7-30367, and provides a SAW device including a SAW resonator that is connected in series to a SAW filter.
In the SAW device disclosed in the above-described publication, as shown in FIG. 38, a SAW resonator 102 is connected in series to a 3-IDT longitudinally coupled resonator SAW filter 101. An IDT is a comb-like electrode portion (also referred to as an xe2x80x9cinterdigital transducerxe2x80x9d).
The frequency of the longitudinally coupled resonator SAW filter 101 and the frequency of the SAW resonator 102 are set such that the antiresonant frequency of the SAW resonator 102 is greater than a pass band of the longitudinally coupled resonator SAW filter 101 and the resonant frequency of the SAW resonator 102 is within the pass band of the SAW filter 101.
With this configuration, since the antiresonant frequency of the SAW resonator 102 is greater than the pass band of the longitudinally coupled resonator SAW filter 101, the attenuation at the high-frequency side close to the pass band is increased. Also, since the resonant frequency of the SAW resonator 102 is within the pass band, a large insertion loss within the pass band is prevented.
In the frequency band between the resonant frequency and the antiresonant frequency of the SAW resonator 102, the impedance is inductive. Thus, by matching this frequency band to a frequency band in which the impedance is capacitive within the pass band of the longitudinally coupled resonator SAW filter 101, the voltage standing wave ratio (VSWR) within the pass band is advantageously decreased.
In the configuration of the known SAW device shown in FIG. 38, however, there is a dip toward the high-frequency side in the pass band, and thus, the pass bandwidth is disadvantageously reduced. The inventors of this application have discovered that the reason for this is due to the generation of a spurious response caused by a surface skimming bulk wave (SSBW) in the frequency band between the resonant frequency and the antiresonant frequency of the SAW resonator 102, which often occurs when a 40xc2x15xc2x0 Y-cut X-propagating LiTaO3 substrate is used.
A typical impedance characteristic of the SAW resonator 102 is shown in FIG. 39, and a phase characteristic thereof is shown in FIG. 40 . In FIGS. 39 and 40, the distortions indicated by A are spurious responses caused by a SSBW (the distortion in FIG. 40 is more noticeable). The generation of a dip toward the high-frequency side in the pass band caused by a spurious response narrows the pass bandwidth, thereby considerably increasing the insertion loss in the pass band due to a temperature change or decreasing the yield due to manufacturing variations.
To overcome the problems described above, preferred embodiments of the present invention provide a SAW device having a wide band while maintaining a large attenuation outside the pass band by shifting a spurious response to a frequency band in which the influence of a dip is negligible.
A SAW device according to a preferred embodiment of the present invention includes a SAW filter provided on a piezoelectric substrate and including at least two IDTs arranged along a direction in which a SAW propagates, and a SAW resonator provided on the piezoelectric substrate and connected in series to the SAW filter. In the SAW resonator, a SSBW is shifted toward a resonant frequency side.
With this configuration, a large insertion loss in the pass band is prevented, and the attenuation outside the pass band very close to the high-frequency side of the pass band is increased. Since the SAW resonator is arranged such that a spurious response caused by a SSBW is shifted to the resonant frequency, a dip occurring at the high-frequency side in the pass band is reduced, thereby increasing the pass band.
A SAW device according to another preferred embodiment of the present invention includes at least one SAW filter provided on a piezoelectric substrate and including at least two IDTs arranged along a direction in which a SAW propagates, and at least one SAW resonator provided on the piezoelectric substrate and connected in series to the SAW filter. The SAW resonator is arranged such that the resonant frequency thereof is located within the pass band of the SAW filter and that the antiresonant frequency thereof is located outside the pass band, and a dummy electrode is provided between an electrode finger and a bus bar of each of the IDTs of the SAW resonator such that a spurious response caused by a SSBW is located within the pass band.
With this configuration, a large insertion loss in the pass band is prevented, and the attenuation outside the pass band very close to the high-frequency side in the pass band is increased. In the SAW resonator, a dummy electrode is provided between an electrode finger and a bus bar of each IDT such that a spurious response caused by a SSBW is located within the pass band. Accordingly, a dip occurring at the high-frequency side in the pass band is reduced, thereby increasing the pass band. The VSWR of the SAW device is also reduced.
In the aforementioned SAW device, a gap between the dummy electrode and the electrode finger of the IDTs of the SAW resonator is preferably about 0.5xcex or less, where the wavelength determined by the pitch between the electrode fingers of the IDTs is indicated by xcex. With this arrangement, a dip occurring at the high-frequency side in the pass band is reduced, thereby increasing the pass band.
In the aforementioned SAW device, the length of the dummy electrode of the SAW resonator is preferably at least about 0.25xcex, where the wavelength determined by the pitch between the electrode fingers of the IDTs of the SAW resonator is indicated by xcex. With this arrangement, a spurious response caused by a SSBW gradually approaches the resonant frequency, namely, the amount by which the frequency of a spurious response is shifted is small. Thus, the characteristics of the SAW device are stabilized even if the lengths of the dummy electrodes vary.
In the aforementioned SAW device, the interdigital length of the IDTs of the SAW resonator is preferably about 45xcex or less, where the wavelength determined by the pitch between the electrode fingers of the IDTs of the SAW resonator is indicated by xcex. With this arrangement, the frequency of a spurious response is adjusted by the dummy electrodes. A dip occurring at the high-frequency side in the pass band caused by the spurious response is reliably reduced, thereby stably increasing the pass band.
In the aforementioned SAW device, the SAW filter is preferably a longitudinally coupled resonator SAW filter. The SAW device is preferably provided with a balanced-to-unbalanced conversion function.
A communication apparatus according to another preferred embodiment of the present invention includes any one of the above-described SAW devices according to other preferred embodiments of the present invention.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.